1. Field of the Invention
The present invention relates to an optical recording medium and optical recording method that switches the irradiation time of a laser beam in many levels in correspondence to data used in the recording, irradiates the optical recording medium with the laser beam and then records the data in multiple levels.
2. Detailed Description of the Related Art
A great deal of research has been carried out relating to methods to record multiple pieces of data in signals with identical length by means of switching the depth of a regeneration signal (modulation factor of reflection signal) in many levels in contrast to methods to record data by means of changing the length of a regeneration signal (length of the modulated part of the reflection signal) in many levels.
According to this optical recording method, because it is possible to record multiple pieces of data in the direction of depth compared to when binary data is recorded depending on the presence or absence of only a pit, the amount of signals assigned to fixed lengths can be increased. Generally, methods to change the depth of a regeneration signal in many levels change the laser beam power in many levels to form different types of recording marks. At present, materials which use holographs or multi-layered recording materials are proposed for the optical recording medium.
Hereupon, a case wherein data is recorded in many levels using a depth variation of the reflectance etc is called multilevel recording.
In this type of multilevel recording the recording marks must be shortened in order to improve the recording density.
Multilevel recording is, however, difficult when attempting to reduce the recording marks smaller than the beam diameter of a converging laser used for recording and reading.
For example, Japanese Patent Laid-Open Publication No. Hei. 10-134353 describes a method in which the quantity of laser light is adjusted in order to record multiple levels. In this method a regeneration signal is formed by differences in the reflection of the recording part and the non-recording part when the recording medium is a dye film or a phase-changing film.
Consequently, in the method disclosed in Japanese Patent Laid-Open Publication No. Hei. 10-134353, the non-recording level and the recording level depend on a relationship of whether or not a recording exists and are not suitable for recording in many levels. Stated more clearly, nothing exists in the intermediate state between recording and non-recording for a phase-changing film or a dye film.
Up to the present the reason why multilevel recording in many levels was possible by means of modulating the quantity of laser light using a dye film or a phase-changing film as the recording medium was mainly due to the fact that the widths of the recording marks were changed by changing the power of the laser.
A converging beam normally forms a Gaussian distribution although when the recording film is a dye film or a phase-changing film, the recording is performed on the portion that exceeds a certain threshold value. Changing the power of the laser changed the spot size of the converging beam that can record which in turn changed the length of the recording marks.
If, however, the length of the recording marks are shortened more than the diameter of the converging beam to increase the recording density, it becomes difficult to perform multilevel recording in many levels, in particular in five levels or more, using a method that modulates the power of the laser to change the recording mark width. In other words, changing the recording power makes it difficult to change the reflection level during regeneration in five levels or more.
Following the conventional principle of gradually switching the laser power to achieve a multilevel recording results in the recording mark length growing larger than the diameter, of the converging beam (beam waist) for recording. Normally, the diameter of the converging beam is expressed by Kλ/NA (K: constant, λ: laser wavelength, NA: numerical aperture of lens). Normal values in a pickup used in a CD are λ=780 nm, NA=0.45 with the diameter of the converging beam being approximately 1.6 μm. For this case, if the recording mark length was set close to 1.6 μm, the above-mentioned problem of signal degradation was actualized which made it difficult to perform multilevel recording in five levels or more using a method that changes the laser power.
Further, there is an example of an optical recording. medium as disclosed in Japanese Patent Laid-Open Publication No. Hei. 1-182846 wherein the absorbance of the reactive material in the recording layer changes as a digital value when a quantity of incident light on the recording layer is supplied as a digital value.
The absolute value of the absorbance change in response to the irradiation amount (frequency) of the laser is presumed to be very small for this optical recording medium however and has not yet attained practical use.
Furthermore, an optical recording method is disclosed in Japanese Patent Laid-Open Publication No. Sho. 61-211835 in which the intensity or irradiation frequency of the irradiation light irradiating a photochromic material is changed in order to record at different arbitrary coloring density states.
In this optical recording method there is a problem in which the coloring density state cannot be read in five levels or more when irradiating and scanning laser light.
The above-mentioned conventional multilevel recording had a problem of worsening signal quality during regeneration as the laser beam power increased during recording, namely, following increases in the depth of the reflection signal to be formed. The reason for this is not clear but a best guess by the inventor is thought to be caused by the area of the recording marks (recording mark area) growing larger due to increases in the laser power.
For example, when performing multilevel recording by shortening, the recording marks and switching the laser power in many levels within those marks in order to increase the density of the amount of recording information on the recording medium, degradation of the signal quality becomes very noticeable and ultimately lead to a state in which the merits of multilevel recording could not be taken advantage of. In other words, when attempting to use multilevel recording, even if the intervals between the recording marks widened and the signal quality degraded, the data had to be reliably detected to a certain degree.
In addition, the recording mark length following the conventional principle of gradually switching the laser power to achieve a multilevel recording is assumed to be larger than the diameter of the converging beam (beam waist) for recording.
The above-mentioned problems are considered to be a result of a complicated combination of all the laser beam power settings and characteristics of the recording medium. As far as the inventor knows, the origin of the problems has not become evident as of the present and high-density multilevel recording is currently not achievable using this recording medium and recording method.